The invention relates to a method of selecting discharge lamps which are each provided with a discharge vessel which supports a luminescent layer at an inner surface, by which method the luminescent layer is excited by means of ultraviolet radiation generated inside the lamp vessel, and the discharge lamps are selected in dependence on the emission spectrum of the light radiated by the luminescent layer. The invention also relates to a device for selecting discharge lamps which are each provided with a discharge vessel which supports a luminescent layer on an inner surface, which device is provided with an excitation source for generating ultraviolet radiation within the lamp vessel and thus exciting the luminescent layer of each of the discharge lamps to be selected and is provided with detection means for detecting the light radiated by the low-pressure mercury discharge lamps to be selected.
A method as mentioned in the opening paragraph is known from German Patent document DE 3911395 C1. Such a method often forms part of a process for recycling low-pressure mercury discharge lamps. Such a process often comprises a process step in which the luminescent layer of the low-pressure mercury discharge lamp (also referred to as "lamp" hereinafter) is removed from the lamp vessel and collected so as to be made suitable for re-use by means of a number of further process steps, or to serve as a basic material for the chemical industry. Widely differing mixtures of luminescent materials, however, are used in the luminescent layers of low-pressure mercury discharge lamps. A mixture of luminescent layers of widely differing compositions is not suitable for re-use. The further process steps for making the materials suitable for re-use, moreover, are not the same for each and every luminescent layer composition. It is often not possible for these reasons to prepare luminescent layer materials jointly for re-use if they originate from low-pressure discharge lamps of different types and made by different manufacturers. A separate recycling of each individual luminescent layer composition, however, would lead to a highly complicated recycling process. This is why it is desirable to achieve a separation into a limited number of groups in the collection of the luminescent layer materials, each group being capable of preparation for re-use by means of its own distinct process steps. It is required for this that the composition of the recycled mixture yielded by recycling of a certain group can be rendered suitable for use in various types of low-pressure mercury discharge lamps in a simple manner, for example through the addition of one or several luminescent substances. The fact that each composition of a luminescent layer of a low-pressure mercury discharge lamp corresponds to a distinct, individual emission spectrum renders it possible to identify this composition through measurement of the emission spectrum of the low-pressure mercury discharge lamp. The excitation of the luminescent layer necessary for this may be realized, for example, by igniting the low-pressure mercury discharge lamp. It is also possible to excite the luminescent layer by inducing a discharge locally in the plasma of the low-pressure 10 mercury discharge lamp by means of an external electric field. If the lamp is leaky, for example, and the composition of the gas filling has been so changed thereby that it is no longer possible to generate a discharge in this gas filling, it is possible to introduce a UV source into the lamp vessel after the latter has been opened at one end and to realize the excitation in this manner. It is possible in principle to derive the composition of the luminescent layer of the low-pressure mercury discharge lamp from the emission spectrum.